Investigation of technological characteristics of Streptococcus thermophilus strains isolated from South Khorasan Masske butter

The objective of present study was to investigate technological properties of nine Streptococcus thermophilus strains isolated from South Khorasan Masske butter. Masske butter is a traditional sheep’s butter in South Khorasan region which Streptococcus thermophilus accounts for 15 percent of its microbial population. After the examination, strains showed positive reactions to following properties: di- acetyl production, proteolytic activity (7 out of 9 strains) excluding two strains, exo-polysaccharide production (8 strains). Also, all strains lost lipolytic activity in tributirin agar and urease activity. Acidification evaluation revealed a noticeable rate of reduction in pH during the first 24 hours, but in second 24- hours, pH reduction rate declined. The highest acidification activity was related to B251 (3.09±0.339) and B272 (3.08±0.1) strains and the lowest one was attributed to Y75 (4.01). Autolysis activity of strains, after 48 hours' incubation, was observed as reduction in optical density. The highest autolytic activity was related to Y106 strain (0.158±0.019) and the lowest one for Y19 (0.446±0.016). In conclusion, it is revealed that excluding y75 and y106, other strains had high proteolytic activity. Also, strain B251 which experienced the highest acidification rate in the first six hours, can be exploited as a suitable strain for starter culture or adjunct (secondary) culture in cheese or yogurt production.